1. Field of the Disclosure
This disclosure relates in general to wellhead assemblies and in particular to seal assemblies for sealing between inner and outer wellhead members.
2. Description of the Related Art
Seals are used between inner and outer wellhead tubular members to contain internal well pressure. The inner wellhead member may be a casing hanger that supports a string of casing extending into the well for the flow of production fluid. The casing hanger lands in an outer wellhead member, which may be a wellhead housing, a Christmas tree, or a casing head. A packoff (or other seal assembly) seals the annulus between the casing hanger and the outer wellhead member. Alternately, the inner wellhead member can be a tubing hanger located in a wellhead housing and secured to a string of tubing extending into the well. A pack off (or other seal assembly) seals the annulus between the tubing hanger and the wellhead housing. In another alternative design, the inner wellhead member may be an isolation sleeve, such as might be used to isolate high pressure, abrasive fracturing fluids from certain portions of the wellhead. A packoff (or other seal assembly) seals the annulus between the isolation sleeve and the outer wellhead member.
A variety of annulus seals of this nature have been employed. Conventional annulus seals include, for example, elastomeric and partially metal and elastomeric rings. Prior art subsea stab type seals may utilize elastomeric materials which are compressed into an interference fit annulus. These are simple designs, easy to install, retain a reasonable constant load when unpressurised over time due to their inherent elasticity and are soft enough to flow and seal on minor defects. However such materials have a limited range of use in terms of temperature and fluid compatibility. They may swell and degrade mechanically in certain fluid environments, such as those found in many wellheads, and can suffer from explosive failure if subjected to rapidly decreasing pressure in a gas environment.
Prior art seal rings made entirely of metal for forming metal-to-metal seals are also employed. In order to cope with internal stressing remote from the interfaces, metal seals of the prior art are made from hard high strength materials which make sealing at the interface difficult and require generating of huge loads to provide any degree of damage tolerance. This in turn will itself cause damage to the same surfaces. To overcome this, coatings in the form of spray coatings or plating or melted inlays, such as brazing, have been used to bond a secondary softer material to the metal seal. These coatings are often difficult to apply, costly, inefficient in material usage, tend to increase the hardness of the sprayed material and are typically difficult to apply thick enough to provide the volume of material required to seal on serious defects.
A third option for the prior art has been to use inelastic thermoplastic materials such as polytetrafluoroethylene, or molded graphite for the sealing apparatus. These do not have any inherent elasticity and so require some secondary parts, such as internal springs, to provide an elastic response to the changing environment that ensures the seal retains a reasonable constant load when unpressurised over time and so ensuring a seal is maintained at all times. Due to the low strength of thermoplastic materials they generally cannot sustain the loads required to cause significant plastic flow at the interface and so do not tend to seal well on damaged surfaces.
Therefore while metal or inelastic materials allow a much wider temperature range, do not swell or degrade mechanically in most fluid environments and do not suffer from explosive gas decompression, they do represent many other technical problems, most notably an inability to seal on damaged surfaces. Damage to subsea parts cannot be fully monitored or controlled and therefore seal failure due to damaged surfaces represents a significant cost risk when running equipment subsea.